text: The FeS ferredoxin from Clostridium pasteurianum (Cp2FeFd; ) has been recognized as distinct protein family on the basis of its amino acid sequence, spectroscopic properties of its iron–sulfur cluster and the unique ligand swapping ability of two cysteine ligands to the [FeS] cluster. Although the physiological role of this ferredoxin remains unclear, a strong and specific interaction of Cp2FeFd with the molybdenum-iron protein of nitrogenase has been revealed. Homologous ferredoxins from Azotobacter vinelandii (Av2FeFdI; ) and Aquifex aeolicus (AaFd; ) have been characterized. The crystal structure of AaFd has been solved. AaFd exists as a dimer. The structure of AaFd monomer is different from other FeS ferredoxins. The fold belongs to the α+β class, with first four β-strands and two α-helices adopting a variant of the thioredoxin fold. UniProt categorizes these as the "2Fe2S Shethna-type ferredoxin" family.